The invention relates to an electron gun for an cathode ray tube, which uses a main lens system designed to focus electron beams emitted from an electron beam generating source.
As is well known, an electron gun for a cathode ray tube (not shown) generally comprises two fundamental sections, that is, a source of electron beams and lens system for focusing electron beams on the fluorescent screen of the cathode ray tube. Customarily, the former is referred to as "a triode-electrode section", and the latter as "a focusing lens (main lens) system". The focusing lens (main lens) system of electron guns for cathode ray tubes used with presently marketed color television receiving sets is mostly of the electro-static focusing type, comprising coaxially arranged conductive tubular elements (electrodes), having predetermined voltages thereacross to produce a desired electric field for focusing electron beams.
Electron guns are generally classified into three types: bipotential types, unipotential types and tripotential types (the latter disclosed in the U.S. Pat. No. 3,995,194).
There will now be described by reference to FIG. 1 the arrangement of electrodes constituting the focusing lens system of the above-mentioned three types of electron guns and the axial distributions of potentials thereof. The bipotential type of electron gun (FIG. 1a) comprises a main lens system formed of two focusing electrodes 1 and 2. The electrode 1 disposed adjacent to the cathode has a relatively low potential. The electrode 2 positioned near the fluorescent screen of the cathode ray tube has a relatively high potential. A potential across these two electrodes consitituting the main lens system has a monotonic or substantially uniformly increasing distribution pattern as illustrated by a curve 3. The axial potential distribution of the electrodes constituting the main lens system of the bipotential type electron gun is referred to as "monotonic", because the first derivative is expressed without changing its sign. However, some of the bipotential type main lens systems of the above-mentioned arrangement have an unsatisfactory spherical aberration characteristic. In a considerably small space like the neck portion of a cathode ray tube provided with the above-mentioned bipotential type electron gun, it is impossible in the case of a particularly high current electron beam to sufficiently reduce the size of a focused electron beam spot, and as a result resolution is not improved.
FIG. 1(b) illustrates the axial potential of a unipotential type main lens system consisting of three focusing electrodes 4, 5 and 6. The axial potential distribution indicates a substantially saddle-shaped pattern as illustrated by a curve 7, with the potentials at the forward and backward ends of the main lens system set at substantially the same level. The unipotential type main lens system arranged as described above indeed, enables a focused electron beam spot to be reduced, but has the drawback that breakdown arises in the cathode ray tube provided with an electron gun comprising said unipotential type main lens system because of a larger potential difference between electrode 4 and the triode-electrode section.
The tripotential type main lens system of FIG. 1(c) is formed of at least three, or preferably four focusing electrodes 8, 9, 10 and 11. The axial potential distribution has a pattern illustrated by a curve 12, which indicates a potential monotonically shifting from a relatively intermediate level to a relatively low level and then monotonically and continuously shifting to a high level.
Unlike the electron guns provided with the bi-potential type main lens system, the electron gun comprising the tripotential type main lens system enables the focused electron beam spot to be satisfactorily reduced. However, the tripotential type electron gun has the drawbacks that a further power source has to be provided to apply a intermediate potential for the focusing electrodes, the length of an electron gun is longer, and it is necessary to assemble a triode-electrode section with extremely high precision.
The second derivative of each of the axial potential distributions of the aforesaid three types of main lens system is denoted by a curve having a positive maximum value as indicated by a curve 3' of a coordinate system given below FIG. 1(a) for the bipotential type; by a curve 7' of a coordinate system shown below FIG. 1(b) for the unipotential type; and by a curve 12' of a coordinate system set forth below FIG. 1(c) for the tripotential type. Therefore, these main lens systems are each referred to as "a single lens".
The performance of an electron gun is primarily expressed by the diameter of an electron beam focused on the fluorescent screen of the cathode ray tube. As generally accepted the smaller said diameter, the more improved the performance of the main lens system, and in consequence the higher the focusing efficiency. In this connection the following description may be given from the standpoint of electron optics. The diameter D.sub.T of an electron beam focused by the focusing lens of an electron gun is shown as follows: ##EQU1## EQU D.sub.X =M.multidot.d.sub.X EQU D.sub.SA =1/2.multidot.M.multidot.C.sub.S .multidot..alpha..sub.O.sup.3
Where:
M: magnification PA1 D.sub.X : diameter of an electron beam depending on the magnification PA1 D.sub.SA : expansion of the diameter of an electron beam resulting from a spherical aberration PA1 D.sub.SC : expansion of the diameter of an electron beam caused by the mutual repulsion of electrons PA1 d.sub.X : diameter of an electron beam at imaginary object PA1 C.sub.S : spherical aberration coefficient PA1 .alpha..sub.O : divergent angle of an electron beam
Accordingly, it is preferred for the main (focusing) lens system of an electron gun that the electron optical magnification (M), the size d.sub.x of imaginary object as observed from the main lens system of an electron gun, the spherical aberration coefficient C.sub.S and the divergent angle .alpha..sub.O of an electron beam as observed from the main lens system of an electron gun are made as small as possible.
A distance between the geometric center of the main lens of an electron gun and a fluorescent screen (an approximate focal length) is determined in relation to the diameter D.sub.X of an electron beam depending on the electron optical magnification degree M, the expansion D.sub.SA of the diameter of an electron beam resulting from a spherical aberration and the expansion D.sub.SC of the diameter of an electron beam caused by the mutual repulsion of electrons. If the distance is fixed the best condition of reducing the diameter of an electron beam as much as possible can be arrived at as illustrated in FIGS. 2(a) and 2(b) for the uni- and bi-potential types of the main lens system of an electron gun respectively. Unless, in this case, the 3-electrode section is properly assembled with the main lens system of an electron gun. The balance due to the resolution of the central portion of a screen, the uniformity of light quantity throughout the central and peripheral portions of the screen, and blooming prominently appeared when very high current will decrease.
Therefore, the electron beam-generating electrode, first and second grid electrodes constituting the triode-electrode section, which define the scattering angle of an electron beam and the position of the spot of an imaginary object, should be spaced from each other with great care. And, the design of the triode-electrode section and that of the main lens system should be examined collectively. The reason is that particularly the prefocus characteristic of an electron gun is determined by the level of voltage applied on those of the electrodes positioned nearest to the cathode side of the main lens system (the electrodes denoted by referential numerals 1, 4 and 8 in FIG. 1) and the inter-electrode distance between the main lens system and the triode-electrode section.
A color television receiving set generally comprises three electron guns. In an electron gun having a bipotential type main lens system, generally the cathode is applied with voltage of about 100 to 150 V; the first grid electrode is supplied with grounding voltage; the second grid electrode with voltage of about 400 to 800 V; the third electrode (first focusing electrode) with voltage of several kilovolts as 4.4 to 5.0 KV; and the fourth electrode (second focusing electrode) having the same potential as the fluorescent screen with voltage of 20 to 30 KV.
The third electrode (first focusing electrode) is about 3.5 times longer than the diameter of the main lens system. An electron beam has a relatively large divergent angle of about 4.degree. to 5.5.degree. as observed from the main lens system. And the third electrode (first focusing electrode) is applied with voltage of several kilovolts as 4.4 to 5.0 KV at most. Therefore, the so-called crossover position appearing at the triode-electrode section varies with a video signal supplied to the first grid electrode or cathode, and tends to give rise to blooming when a particularly high current is introduced. As compared with electron optical magnification of an electron gun comprising a unipotential type main lens system, the magnification of an electron gun having a bipotential type main lens system is smaller, so that degree of resolution with respect to the so-called low brightness when a low current is introduced is higher. However, the bipotential type electron gun has the drawbacks that an electron beam has a large divergent angle, resulting in a decline in the uniformity of focusing and a prominent appearance of blooming.
In contrast, in an electron gun having an unipotential type main lens system the third electrode (first focusing electrode) and fifth electrode (third focusing electrode) are applied with high voltage (or the same voltage as that applied to a fluorescent screen), and the fourth electrode (second focusing electrode) is supplied with a substantially zero volt or several kilovolts. Consequently, as the divergent angle of an electron beam is as small as about 2.degree., a more satisfactory uniformity of light quantity is attained throughout the central and peripheral portions of a screen, and the appearance of blooming is suppressed more effectively. However, the electron optical magnification is slightly larger than in a bipotential type main lens system, thereby the resolution is somewhat lower at the time of low brightness when a low current is introduced.
The tripotential type focusing lens system is disclosed in the U.S. Pat. No. 3,995,194. Where a difference between the maximum and minimum values of the second derivative V.sub.o " of the distribution of the potential V.sub.o is reduced as much as possible in the region where the potential distribution in the axial direction of the focusing lens system has a small value or in the region where electron beam collectively has a large diameter, then it is possible to obtain a required focusing force, thereby restricting spherical aberration. The tripotential type focusing lens system comprises a third electrode (first focusing electrode) of FIG. 1(c) (8), a fourth electrode (second focusing electrode) of FIG. 1(c) (9), a fifth electrode (third focusing electrode) of FIG. 1(c) (10), and a sixth electrode (fourth focusing electrode) of FIG. 1(c) (11). The third (first focusing electrode) and the fifth electrode (third focusing electrode are applied with a relatively intermediate voltage of 10 to 12 KV; the fourth electrode (second focusing electrode) with a relatively low voltage of 5 to 7 KV; and the sixth electrode (fourth focusing electrode) with the potential of a fluorescent screen of 20 to 30 KV. The axial potential distribution of the tripotential type focusing lens system shows a pattern smoothly and monotonically shifting from a relatively intermediate potential level to a relatively low-potential level, further smoothly and monotonically shifting to high potential level.
With the tripotential type focusing lens system, the constituent focusing electrodes should have such arrangement and the voltage applied thereon should have such level that the aforesaid axial potential distribution acts as one focusing lens.
To this end, the lengths of the respective focusing electrodes normalized by the diameter of the main lens system are determined to be 0.5 to 2.2 for the fourth electrodes (second focusing electrode) of FIG. 1(c) (9) and 0.75 or less for the fifth electrode (third focusing electrode) of FIG. 1(c) (10). On the other hand, the length of the third electrode (first focusing electrode) of FIG. 1(c) (8) is determined primarily by the geometric dimensions of a color cathode ray tube and a value of voltage applied thereto. An electron gun comprising a tripotential main lens system is characterized in that the focusing electrodes having the main lens system are supplied with the two higher voltages (6 or 7 KV and 10 to 12 KV) than these applied to the customary electron gun having of a bipotential type main lens system to increase the degree of the electron optical magnification; a necessarily occurring rise in the spherical aberration is positively suppressed by converting said lens system into the aforesaid integral form. Consequently the resolution of a low brightness region is prominently improved (because a spherical aberration in a region of relatively low current is negligibly small); and since the third electrode (first focusing electrode) of FIG. 1c (8) is impressed with a relatively high level of voltage (10.about.12 KV), blooming is noticeably eliminated.
On the other hand, the focusing electrodes of the main lens system are elongated and, though the third electrode (first focusing electrode) of FIG. 1(c) (8) is impressed with a relatively high level of voltage, it fails appreciably to reduce the divergent angle of an electron beam as observed from the main lens system owing to the elongated focusing electrodes of the main lens system.
The electron gun formed of the tripotential main lens system has further disadvantages that where said electron gun is used with a self-concentration type color cathode ray tube which has recently become the leading type on the market, though the focus is very satisfactory in the central portion of a picture, the electron beam is subject to a deflection aberration caused by a very astigmatic magnetic field, so that the uniformity of the focus decreases; and the spherical aberration is affected by the deflection aberration to give rise to a spot distortion, leading to a further decrease in the uniformity of the focus.
Recent developments in color cathode ray tubes have improved the focusing characteristic of an electron gun. This improvement implies the formation of a smallest possible electron beam spot on a fluorescent screen. With the prior art main lens system, this requirement has to be met by the extremely high precision with which electron guns are assembled.
As is well known, the neck portion of a color cathode ray tube is provided with a color purity correction magnet and a convergence correction magnet in order to compensate for various errors in assembling the color cathode ray tube including an electron gun.
Where an attempt is made to improve the performance of an electron gun, the expansion D.sub.SA of the diameter of an electron beam depending on the spherical aberration becomes noticeable. Where, in such case, the assembling error of a cathode ray tube is corrected by the aforesaid magnets, so-called smudges tend to appear even in the central portion of a screen owing to the spherical aberration and spot distortion and the focusing characteristic of an electron gun decreases. Therefore, electron guns should be assembled with as high precision as possible.
The spherical aberration and spot distortion depend on the divergent angle of an electron beam emitted from its source, and consequently the unipotential type main lens system does not raise substantially great problems, but the bi- and tri-potential type main lens systems present considerable difficulties.